In this investigation induction and selection of temperature- sensitive mutants is used as a method for probing cellular development in a unicellular eukaryotic organism, Tetrahymena pyriformis. Studies to date have resulted in the isolation and genetic analysis of five mutants blocked in specific stages of the fission process and one mutant with altered cell shape but unmodified cortical pattern. In additional putative mutant an alteration in shape is a consequence of switching over from the cell division pathway to an alternative oral replacement pathway. In these mutants nuclear and cytoplasmic developmental processes are variously dissociated. A model of interrelations of cell cycle functions can be constructed on the basis of studies of these mutants as well as certain other information. Proposed further studies on the existing mutants are designed with threefold objectives: (1) the testing of predictions based on the cell-cycle model, with consequent refinement of the model; (2) analysis of the times and modes of action of the altered gene products, and (3) the uncovering of underlying rules of positional information controlling intracellular pattern. The last objective is approachable because the Tetrahymena cell has complex ciliary patterns that become variously disordered in mutant cells cultivated at high temprature. In addition, further mutagenesis is planned, with selection geared to the isolation of mutants specifically arrested in different components of the fission process and also breedable mutants affected in the "pathway switch" between cell division and oral replacement.